The invention relates to inverter circuits in general, and more particularly to inverters using transistors for power conversion.
A background teaching of transistor characteristics in power conversion as compared to controlled rectifiers can be found on pages 32, 33 of a book entitled "Principles of Inverter Circuits" by B. D. Bedford and R. G. Hoft published in 1964 by John Wiley and Sons. The capability to turn-on power, to turn-off power and the switching speed have been stressed in this book as the major differences existing between these two types of switching devices. While it is desirable, when using a power transistor in the switching mode, to avoid the linear region in order to reduce power dissipation in the transistor, the base current is required to flow during the entire period of conduction and at a level sufficient to maintain saturation. In theory, the base is driven by current which is zero at the instant of cut-off, and which is restored to the base at the proper level when the transistor should become saturated again. In practice, transistors are used in pairs, one for each polarity of the DC power supply, and the base current is supplied from an AC current source having the frequency of the desired AC load. A known type of transistor switch inverter can be found on pages 48, 49 of the aforementioned book of Bedford and Haft.
Commutation is a well recognized problem in inverter circuits. Special commutation circuitry has been designed in order to reduce the current to zero at the moment of cutting off and to postpone impressing again the forward voltage until the switch has completely recovered, otherwise there would be a return to conduction at an inappropriate time. However, such circuitry has inherent limitations to increased switching speed and power output.
It is an object of the present invention to provide an improved inverter circuit of the type using transistors as the power switches.
Another object of the present invention is to obtain an inverter circuit using power transistors which are free from shoot-through between the terminals of the DC power supply.
A further object of the present invention is to provide an inverter circuit of the transistor switch type having an improved capability of generating highfrequency and high power output.